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Despite the fact that benzene (Bz) and pyridine (Py) are probably the most common and extensively studied organic molecules, the observation of a covalent adduct in the ionized benzene/pyridine system has never been reported. This Article reports the first experimental and theoretical evidence of a covalent (Bz x Py)(*+) adduct that results from the reaction of Bz(*+) with pyridine or Py(*+) with benzene. These reactions are studied using mass-selected ion mobility, chemical reactivity, collisional dissociation, and ab initio calculations. The (Bz x Py)(*+) adduct does not exchange ligands with Bz to form Bz(2)(*+) or with Py to form (Py)(2)H(+) despite the strong bonds in these homodimers. The thermochemistry then suggests that the (Bz x Py)(*+) heterodimer is bonded covalently with a bonding energy of>33 kcal/mol. Correspondingly, ab initio calculations identify covalently bonded propeller-shaped isomers of (Bz x Py)(*+) with bonding energies of 31-38 kcal/mol, containing a C-N bond. The mobility of the (Bz x Py)(*+) adduct in helium is consistent with these covalent dimers. As to noncovalent adducts, the computations identify novel distonic hydrogen-bonded complexes (C(5)H(5)NH(+) x C(6)H(5)(*)) where the charge resides on one component (PyH(+)), while the radical site resides on the other component (C(6)H(5)(*)). Collisional dissociation suggests that the covalent and distonic dimers may interconvert at high energies. The most stable distonic (C(5)H(5)NH(+) x C(6)H(5)(*)) complex contains a hydrogen bond to the phenyl radical carbon site with a calculated dissociation energy of 16.6 kcal/mol. This bond is somewhat stronger than the NH(+) x pi hydrogen bonds of PyH(+) to the pi system of the phenyl radical and of the benzene molecule. For this NH(+) x pi bond in the PyH(+) x Bz dimer, the measured binding energy is 13.4 kcal/mol, and ab initio calculations identify two T-shaped isomers with the NH(+) pointing to the center of the benzene ring or to the negatively charged C atoms of the ring. In contrast, the more stable proton-bound PyH(+) x Py dimer contains a linear NH(+)...N hydrogen bond. The formation of the (benzene/pyridine)(*+) adduct may represent a general class of addition reactions that can form complex heterocyclic species in ionizing environments.

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The mechanism of the sulfonation of toluene has been explored both experimentally and theoretically using molecular orbital methods. Sulfonation with sulfur trioxide is proposed to proceed initially via the formation of a toluene-S 2 O 6 π-complex (3) which rearranges to form a Wheland pyrosulfonate intermediate (5) which in turn undergoes a facile prototropic rearrangement involving the transfer of the ring hydrogen at the sp 3 carbon to the sulfonate oxygen atom to form toluenepyrosulfonic acid (7). Once formed, this acid is thought to attack toluene to form two equivalents of toluenesulfonic acid (6) which preferentially react with sulfur trioxide to re-form the pyrosulfonic acid (7). Experimentally, sulfonation using either acetylsulfonic acid (9), trifluoroacetylsulfonic acid (10), or trimethylacetylsulfonic acid (11), as models for pyrosulfonic acid ( 7), appears to show second order kinetics at room temperature. The reaction with acetylsulfonic acid (9) shows no significant kinetic isotope effect when 4deuterotoluene is used as the substrate, suggesting that sulfonation proceeds via attack of the π-electrons of the toluene ring at the sulfur atom, S8, of acetylsulfonic acid or toluenepyrosulfonic acid with simultaneous cleavage of the O7-S8 bond, where the displaced acetate or toluenesulfonate anion respectively can facilitate the removal of the ring proton at the sp 3 carbon.